Sizing Your Solar SystemThe steps that you should follow to
design the right solar system for your home orbusiness include the following:1. Determine Your Power Consumption
Demands - Make a list of the
appliancesand/or loads that you are going to run
from your solar power system. Start your listwith three columns, one for the
appliance name, one for the wattage, and one forcurrent type (AC or DC). Find out how
many watts each device uses and whether itruns on AC or DC power. Most appliances
have a label on the back that lists thewattage. Specification sheets, local
appliance dealers, and product manufacturers arealso good sources of this information.
If you can not find the wattage but you doknow the volts and amps, simply multiply
the volts by the amps to determine thewattage (Volts X Amps = Watts).Step 1 Example

Appliance Name

Watts

Current

Television

100

AC

Lights

60

AC

Laptop Computer

35

DC

2. Optimize Your Power System Demands
- At this stage, it is important toexamine your power consumption and
reduce your power needs as much as possible(the cost savings can be substantial).
First, identify large and/or variable loads (suchas water pumps, outdoor lights, electric
ranges, AC refrigerators, clothes washers,etc.) and try to eliminate them or
examine alternatives such as propane or DCmodels. Also, replace incandescent light
bulbs with fluorescent lights whereverpossible (they provide the same amount
of illumination, but at lower wattage levels.After you've completed this process, you
have your optimized load list.3. Size Your Inverter -
We'll do an easier one first. Take a look at the ACappliances
that are on your optimized load list. Add together the wattage of all
of theappliances that must/will run at
the same time. This is your rough inverter size. Wesuggest
that you add 25% to this number to allow your system to grow. Whenyou've completed this step, you should have your
wattage total + 25%. Sincemanufacturers
measure the size of their inverters in watts, your total tells you thesize of the inverter required.Step 3 Example

Appliance Name

Watts

Current

Television

100

AC

Lights

60

AC

Laptop Computer

35

DC

TOTAL AC

160

(160 X .25 ) +160 = 200 WATTS
(Inverter Size)4. Size Your Battery Bank
- Sizing your battery bank is a little harder. We'll giveyou a relatively simple method to
develop a rough estimate of your needs.A. Add an additional column to
your optimized load list that you developed in step 2 and callit
Hours of Daily Usage. In this column you are going to estimate the
number of hoursyou expect to use your
appliance each day. For example, if I expect to watch my televisionfor 4 hours a day, I place a 4 in the new column.
If I expect to use a laptop computer for anhour
and a half I would enter 1.5 into the new column.B. After you've completed your
estimates for all of the items on your list, add a secondcolumn
called Daily Watt Hours Used. In this column, multiply the Watts by
the Hours ofDaily Usage for each row in
your list. Finally, add the Daily Watt Hours Used column todetermine
the Total Daily Watt Hours Used.C. Now, multiply the Total Daily
Watt Hours Used by the anticipated number of consecutivecloudy
days expected or the number of days of autonomy you would like your
system tohave (i.e. days between charging
- usually between 1 - 5) to determine your RoughBattery
Size in Watt Hours.D. Multiply your Rough Battery Estimate
by 2 to determine your Safe Battery Size inWatt Hours. This allows for 50%
maximum battery discharge during normal operations,and
provides for an additional 50% for emergency situations.E. Now we need to determine your Safe
Battery Size in Amp Hours by converting fromSafe Battery Size in Watts. To do
this you simply divide your Safe Battery Size in Watts byyour
Expected System Voltage (12, 24, or 48 volts DC).F. Determine the Battery Amp Hour
Rating and the Battery Voltage for the battery thatyou
would like to use in your system.G. Divide the Safe Battery Size
(from step E) in Amp Hours by the Amp Hour Rating (stepF)
for your chosen battery. You now know the Number of Batteries in
Parallel.H. Divide the Expected System
Voltage by the Battery Voltage (step F). You now know theNumber
of Batteries Serial.I. Multiply the number of
batteries that you will need to wire in parallel by the number ofbatteries in series to determine Total Number of
Batteries in Your Battery Bank.Step 4 Example

Appliance Name

Watts

Current

Hours ofDaily Usage

Daily WattHours Used

Television

100

AC

4.0

400

Lights

60

AC

4.0

240

Laptop Computer

35

DC

1.5

52.5

Total Daily Watt Hours Used

692.5

C. Number of Days of Autonomy = 3C. Rough Battery Estimate = 3 X 692.5 =
2,077.5D. Safe Battery Size in Watt Hours = 2 X
2,077.5 = 4,155E. Safe Battery Size in Amp Hours =
4,155/12 Volts = 346.25 AHF. Selected Battery Characteristics =
12V, 60AHG. Number of Batteries in Parallel =
346.25/60 = 6H. Number of Batteries in Serial = 12/12
= 1I. Total Number of Batteries in Bank
(supports the load for three days withoutcharging) = 1 X 6 = 65. Determine the Sun Hours Available
Per Day - Another easy one.
For India,you can take 5-1/2 hours.6. Size Your Array -A. Take your Total Daily Watt
Hours Used in step 4-B and multiply by seven. Thisis
your Total Weekly Watt Hours Used.B. Multiply this number by 1.2 to
correct for inverter loss.C. Divide the result by 12 or 24 (12 for
12 volt systems). You now have your TotalAmp Hours Per Week.D. Multiply this number by 1.2 to
compensate for loss from batterycharge/discharge.E. Divide this number by the average Sun
Hours Available Per Day (from step 5).This is your total solar amps required
to support your electrical load.F. Divide this number by the optimum or
peak amps of the solar module you intendto use. Round-off to the next highest
whole number. This is the number of modulesyou need wired in parallel.G. If your system is 12V you don't need
any modules wired in series, 24V you need 2modules wired in series, 48V you need 4
modules wired in series, assuming thenominal panel voltage of 12 V.H. If your system is 2V4 or 48V,
multiply the number of modules in step F by thenumber of modules in step G. This is the
total number of modules required in yoursystem. If your system is 12V use the
number from step F to determine your totalnumber of modules.7. Size your Charge Controller -
Your controller must be able to handle themaximum currrent produced by the PV
modules; as well as the maximum loadcurrent, if the controller includes load
control. Clouds and reflected sunlight fromsnow or building surfaces can increase
the amount of PV output current by as muchas 25% above the modules short circuit
current rating. The controller must be sizedto handle this current rating.
Underwriters Laboratories (UL) and the NationalElectrical Code (NEC) require that the
controller is oversized by an additional 25% sothat the controller does not constantly
operate at 100% of its rating. Therefore, thecontroller needs to be able to handle at
least 156% of the module's (or modules', ifwired in parallel) short circuit current
rating.For Example:12Volt DC SystemArray Size: 2 - 75 watt solar panels
(short circuit current - 4.8 amps each)Modules wired in parallel:4.8 amps x 2 = 9.6 amps9.6 amps x 1.56 = 14.97 ampsSo, a 15 amp controller or larger will
do the job. (Don't forget to allow for expansionof the system - it is cheaper in the
long run to buy a bigger controller that canaccommodate an extra PV module or two.)8. Size your System Interconnects -
Choose the appropriate size wires/cables toconnect your solar modules, batteries,
and inverters. Consult our wire sizing page fordetails.9. Buy products from Sadhana Energy
Devices (This step is not mandatory, but itdoes helps us maintain and expand our
information pages!)10. Install Your System Safely - Make
sure you have the knowledge and skillsnecessary to install your system without
hurting yourself or damaging your property.Be sure to comply with all applicable
codes in your area.